How can I remove the ability to reset passwords outside of Ubuntu. This probably is ill advised, but I already have set a really good password that I cannot forget with high entropy and 42 characters. I think that it is great that Ubuntu can so easily reset a password, but it seems like a big security risk. (I know you can edit grub, but anyone that knows anything about Linux will just edit the boot command and boot into recovery anyway.) Thanks in advance.
Disable the recovery mode by uncommenting the line:
in /etc/default/grub (and running
sudo update-grub afterwards), and password protect grub menu entry.
The only way to be sure is to encrypt the disk. Short of that, you have to physically secure the machine so that someone can not remove the HD and plug it into another machine or boot the machine from removable media. Only then does locking down grub do any good.
The best way to protect your installation is to apply all the above fixes and lock the PC into a box with ventilation in a room that only you have a key to.